not possible currently. it is possible to track usage based on metadata, which we already display on our control plane dashboard but not as an enforcement of limits.
if your use case is to keep limits at per user level, you can create api key per user and set budgets to it. Ps: Budget limits are part of our enterprise offering
Also, @Tim we've pushed out dynamic usage settings on the product recently (dynamic rate limits on requests/tokens etc), and next on the cards is metadata-based rate limiting, exactly what you described!